1. Field of the Invention
The present invention relates to a thermal-type infrared detection element having an improved infrared absorption film that constitutes a photoreceptor.
2. Description of the Related Art
A thermal-type infrared detection element measures the temperature of an object from the change in resistance that occurs in a heat-sensitive resistor. This usually occurs when infrared rays emitted by a body are absorbed and converted to heat in an infrared absorption film, the temperature of a bolometer thin film or other heat-sensitive resistor that forms a diaphragm having a microbridge structure is increased, and the resistance of the resistor is changed.
More specifically, this type of thermal-type infrared detection element is composed of a photoreceptor and a beam for holding the photoreceptor in a suspended state above a circuit board. The photoreceptor is provided with a bolometer layer and an infrared absorption film for absorbing incident infrared rays and protecting the bolometer layer. The beam is provided with wiring for connecting the bolometer layer with a reading circuit formed in advance on the circuit board. When incident infrared rays are absorbed by the infrared absorption film, and the temperature of the photoreceptor is increased, the resistance of the bolometer layer changes, and this change in resistance is detected by the reading circuit and outputted as a temperature. A thermal-type infrared detection element having this type of structure is disclosed in JP-A 2002-71452 (pp. 5-8, FIG. 6), for example.
Increasing the change in resistance of the bolometer layer with respect to the change in temperature of the photoreceptor is of primary importance in increasing the sensitivity (S/N ratio) of the thermal-type infrared detection element described above. Therefore, a material having a large temperature coefficient of resistance (TCR: Temperature Coefficient Resistance) is used for the bolometer layer. Increasing the efficiency with which incident infrared rays are absorbed is the second most important factor. In order to achieve this object, an infrared reflection film is provided in a position facing the photoreceptor on the circuit board, and the gap between the photoreceptor and the infrared reflection film is set so that an optical resonance structure is formed. In order to also increase the absorption efficiency with respect to incident infrared rays, the infrared absorption film that constitutes a photoreceptor is endowed with a laminate structure made up of films composed of different materials.
Since the thermal-type infrared detection element thus configured detects infrared rays having wavelengths from 8 to 14 μm in the so-called atmospheric window, the infrared absorption film must be composed of a material that absorbs infrared rays having the abovementioned wavelengths. From a manufacturing standpoint, the infrared absorption film must also be composed of a material that is easily formed into a film, etched, or otherwise processed. Therefore, silicon oxide (SiO), silicon nitride (SiN), silicon carbide (SiC), silicon oxynitride (SiON), silicon carbonitride (SiCN), and the like have conventionally been used as materials that satisfy these conditions.
However, although these materials absorb infrared rays in the abovementioned waveband, since the absorption rate is high on the long-wavelength end (approximately 10 to 13 μm) of the abovementioned waveband, and is low on the short-wavelength end thereof (approximately 8 to 10 μm), infrared rays on the short-wavelength end of the abovementioned waveband are not effectively utilized.
More specifically, as shown in FIG. 1, SiC has maximum absorption of wavelengths near 13 μm; SiN, SiCN, and SiON have maximum absorption of wavelengths near 12 μm; and absorption is high for infrared rays on the long-wavelength end of the abovementioned waveband. However, absorption sharply declines on the short-wavelength end, and the absorption in the waveband of 8 to 12 μm for SiON and SiN is only about 20%. Since the absorption rate of SiO is at maximum near 10 μm, but is generally low compared to that of SiN, SiON, and SiC, infrared rays having wavelengths in the abovementioned range cannot be efficiently absorbed. Infrared rays on the short-wavelength end of the abovementioned waveband therefore cannot be effectively utilized even when infrared absorption films composed of these materials are combined in a laminate. This results in drawbacks whereby the sensitivity of the thermal-type infrared detection element cannot be adequately increased.
A method for increasing the thickness of the infrared absorption film is also considered as a way of increasing the absorption efficiency of infrared rays, but the heat capacity of the photoreceptor increases when the thickness of the infrared absorption film is increased, and the temperature change-with respect to the incident infrared rays decreases. Since an infrared absorption film is also formed in the beam, the beam increases in diameter, and the amount of heat discharged towards the circuit board increases. Therefore, the sensitivity of the thermal-type infrared detection element cannot be increased by this method.